Forget top speed.
Focus on your WOT (in gear) rpm and your engine’s rated rpm. The rpm the engine runs when it develops it’s rated power.
Then your top rpm in gear should be the rating rpm to about 150 more. 100rpm over rated rpm is optimum IMO.
I just changed props and I’m getting rated rpm (3000) and she runs perfect. And she marches right up to it. Unlike before where the last 100rpm was slow to max out. 2500 (top cruising rpm) smooth and easy going. 2300 smooth and easy w a little less noise. Just a slight shake at a spot or two well below cruising rpm. Perfect or as perfect as I’ll ever get IMO.
My old prop was 125rpm below rated. Overpropped. I know this is a bit subjective but the whole propulsion system seemed a tiny bit stressed. And a case could be made that it actually was.
I’m very happy w the way she runs now.

I'd partially agree with the others that WOT RPM and as Eric said plus 150 or so is the more important figure, your top speed is handy to know as well. In emergency situations it would be nice to know what speed you can do to get there or away fast. Also it can give you and idea of how much fouling the vessel has if you give her a run at WOT every now and then which some people would argue is good for the engine as well.

Sorry I have no idea what you should be getting as I don't know the hull at all but reviews on the same hull with a 385hp Cat suggest about 17knots so maybe 18 for your Yanmar.

You can check your engine WOT specs online if you have the engine model number, Brett

I have been out once this year because of health reasons.The boat was bottom painted in march and engine taken care of. I got a little over 3300 rpms.the boat made it to 15 knots it just seemed slower then last year. Tom

Only run that engine at top speed for a few minutes once or twice a year to see what its wot rpm is. At a little over 3,300 rpm that is too low if you ever plan to run fast for any length of time. Need to have pitch adjusted to reach 3,500.

That engine is very sensitive to load, ie prop pitch. Can run 2,800 for extended period if wot rpm is 3,500. Any more (or less wot pitch) and they have blown.